It's inside you, dummy!

I'm filling in for my boss at work, in the middle of a big project, while she goes to France. More money, but also more responsibility. I confessed to a colleague at work that I'm not enjoying myself. She berated me, saying I'll build up negativity and be even more miserable. And she's right, of course. I learnt from Pouch that it's all about attitude (or, in my case, rattitude) and my personal life's changed. So why can't I do the same in my professional life?